1. Field of the Invention
The present invention relates to a surface-mount quartz crystal oscillator having a quartz crystal unit and an IC (Integrated circuit) chip having an oscillating circuit which uses the crystal unit, and more particularly to a surface-mount crystal oscillator which accommodates electronic components other than an IC chip and is of a small overall height.
2. Description of the Related Art
Since surface-mount crystal oscillators are small in size and weight, they are widely used as a frequency or time reference source in portable electronic devices. In view of the trends toward smaller electronic devices, efforts are being made to achieve more reductions in planar profile dimensions and height of surface-mount crystal oscillators. One type of a surface-mount crystal oscillator accommodates an IC chip in a space separate from a crystal unit.
FIG. 1A shows by way of example a conventional surface-mount crystal oscillator. As shown in FIG. 1A, the conventional surface-mount crystal oscillator comprises crystal unit 1 and mounting substrate 2. Crystal unit 1 has casing 3 having a recess defined therein, quartz crystal blank 4 disposed in the recess, and cover 5 of metal placed on casing 3 and hermetically sealing crystal blank 4 in the recess. Cover 5 is joined by seam welding, for example, to the upper surface of a frame wall which surround the recess in casing 3.
Connecting terminals 6 are disposed on the respective four corners of an outer bottom surface, i.e., a reverse surface, of casing 3. Connecting terminals 6 include a pair of crystal connecting terminals electrically connected to crystal blank 4. Connecting terminals 6 also include a ground terminal electrically connected to cover 5, for example. A pair of connecting electrodes (not shown) is formed on the bottom surface of the recess in casing 3 and electrically connected to the crystal connecting terminals through electrically conductive paths on casing 3.
Crystal blank 4 comprises a substantially rectangular AT-cut quartz crystal blank, for example. Excitation electrodes (not shown) are formed respectively on the principal surfaces of crystal blank 4, and extension electrodes (not shown) extend from the respective excitation electrodes to opposite sides of one end of crystal blank 4. The sides of the end of crystal blank 4 to which the extension electrodes extend are fixed to the connecting electrodes formed on the bottom surface of the recess in casing 3 by electrically conductive adhesive 7. Crystal blank 4 is thus held horizontally in casing 3 and electrically and mechanically connected thereto.
Mounting substrate 2 is made of laminated ceramics and has a substantially rectangular elongate planar shape with a recess defined in one principal surface thereof. Actually, mounting substrate 2 comprises bottom wall 8 in the form of a substantially rectangular plate and frame wall 9 in the form of a substantially rectangular frame. Frame wall 9 serves as a side wall of the recess. Connecting terminals 11 are arranged on the upper surface of frame wall 9 which surrounds the recess in alignment with respective connecting terminals 6 on casing 3. Connecting terminals 6 on casing 3 and connecting terminals 11 on mounting substrate 2 are electrically and mechanically connected to each other by, for example, soldering, thus completing the surface-mount crystal oscillator. Mounting terminals 10 which include terminals such as a power supply terminals, a ground terminal, and an output terminal are disposed on the four corners of an outer bottom surface of bottom wall 8. Mounting terminals 10 are used to mount the surface-mount crystal oscillator on a wiring board.
IC chip 12 is fixed to the bottom surface of the recess in mounting substrate 2 by ultrasonic thermo-compression bonding or thermo-compression bonding with bumps 13, for example, interposed therebetween. IC chip 12 is electrically connected to mounting terminals 10 and connecting terminals 11 by electrically conductive paths on mounting substrate 2. Frame wall 9 has cavities defined in surfaces thereof facing the recess in mounting substrate 2, and chip capacitors 14 are accommodated in the respective cavities, as shown in FIG. 1B. Chip capacitors 14 are bypass capacitors having a large capacitance, for example, which are difficult to integrate in IC chip 12. Chip capacitors 14 are electrically connected to IC chip 12.
To protect the surface of IC chip 12 which is joined to bumps 13, resin 15 is poured and solidified in the recess in mounting substrate 2 to the extent that IC chip 12 is embedded in resin 15 up to about half of the thickness thereof.
In view of the tendency in the art to reduce the size of surface-mount crystal oscillators of the above structure, the recess in mounting substrate 2 has its internal dimensions reduced, allowing only IC chip 12 to be housed therein and making it difficult to accommodate chip capacitors 14 even in the presence of the cavities in frame wall 9. With IC chip 12 and chip capacitors 14 being installed in the recess in mounting substrate 2, since chip capacitors 14 are generally of a greater height than IC chip 12, it is difficult to reduce the height of the surface-mount crystal oscillator because of limitations posed by the height of chip capacitors 14.